1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of performing hybrid automatic repeat request (HARQ) in a wireless communication system.
2. Related Art
An electrical and electronics engineers (IEEE) 802.16 wireless communication system provides a high-speed multimedia communication service to users in addition to the conventional voice and packet data communication services. The multimedia communication service is a service for transmitting data packets to a plurality of user equipments (UEs), and is referred to as various terms such as a broadcast/multicast service, a multimedia broadcast multicast service (MBMS), a multimedia and broadcast service (MBS), and a point-to-multipoint service. These terms are used without distinction in the following description.
The broadcast/multicast service is based on Internet protocol (IP) multicast. In this service, UEs receive the same multimedia data by sharing resources required for data packet transmission. Therefore, if a certain number of UEs using the broadcast/multicast service exist in the same cell, resource efficiency can be increased. The broadcast/multicast service operates in two modes, that is, a broadcast mode and a multicast mode. In the broadcast mode, one transmitter transmits data to all receivers on the same network. In the multicast mode, one or more transmitters transmit data to one or more specific receivers. In the multicast mode, an intention to receive data has to be delivered to a network, and group registration and deregistration are supported.
Meanwhile, an error compensation scheme is used to secure communication reliability. Examples of the error compensation scheme include a forward error correction (FEC) scheme and an automatic repeat request (ARQ) scheme. In the FEC scheme, errors in a receiving end are corrected by attaching an extra error correction code to information bits. In the ARQ scheme, errors are corrected through data retransmission. Examples of the ARQ scheme include a stop and wait (SAW) scheme, a go-back-N (GBN) scheme, a selective repeat (SR) scheme, etc. The SAW scheme transmits a frame after determining whether the transmitted frame is correctly received. The GBN scheme transmits N consecutive frames, and if transmission is unsuccessful, retransmits all frames transmitted after an erroneous frame. The SR scheme selectively retransmits only the erroneous frame.
The FEC scheme has an advantage in that a time delay is small and no information is additionally exchanged between a transmitting end and a receiving end but also has a disadvantage in that system efficiency deteriorates in a good channel environment. The ARQ scheme has an advantage in that transmission reliability can be increased but also has a disadvantage in that a time delay occurs and system efficiency deteriorates in a poor channel environment. To solve such disadvantages, a hybrid automatic repeat request (HARQ) scheme is proposed by combining the FEC scheme and the ARQ scheme. In the HARQ scheme, it is determined whether an unrecoverable error is included in data received by a physical layer, and retransmission is requested upon detecting the error, thereby improving performance.
A receiver using the HARQ scheme basically attempts error correction on received data, and determines whether the data will be retransmitted or not by using an error detection code. The error detection code may be a cyclic redundancy check (CRC). When an error of the received data is detected in a CRC detection process, the receiver transmits a non-acknowledgement (NACK) signal to a transmitter. Upon receiving the NACK signal, the transmitter transmits relevant retransmission data according to an HARQ mode. The receiver receives the retransmission data and then performs decoding by combining the retransmission data with previous data. As a result, reception performance is improved.
The HARQ mode can be classified into a chase combining mode and an incremental redundancy (IR) mode. In the chase combining mode, to obtain a signal-to-noise ratio (SNR), error-detected data is combined with retransmitted data instead of discarding the error-detected data. In the IR mode, additional redundant information is incrementally transmitted with retransmitted data to reduce an overhead resulted from retransmission and to obtain a coding gain.
According to a transmission attribute, the HARQ can be classified into an adaptive HARQ and a non-adaptive HARQ. The transmission attribute includes resource allocation, a modulation scheme, a transport block size, etc. In the adaptive HARQ, depending on changes in a channel condition, transmission attributes are entirely or partially changed by comparing transmission attributes used for retransmission with transmission attributes used for initial transmission. In the non-adaptive HARQ, the transmission attributes used for the initial transmission are persistently used irrespective of the changes in the channel condition.
A HARQ-based retransmission scheme can be classified into a synchronous HARQ and an asynchronous HARQ. The synchronous HARQ retransmits data at a time point known to the transmitter and the receiver. In the synchronous HARQ, signaling required to transmit data such as a HARQ processor number can be reduced. The asynchronous HARQ allocates resources for retransmission at any time point. In the asynchronous HARQ, an overhead occurs due to signaling required for data transmission.
In the process of performing HARQ, a UE may receive different types of data packets. For example, in the process of performing HARQ, the UE may receive an MBMS from a base station (BS). In this case, a problem may arise in that the MBMS collides with ACK/NACK transmission or data retransmission.